Frequency modulation system



June 1950 R. G. SCHRIEFER 2,510,144

- FREQUENCY MODULATION SYSTEM Filed Feb. 1, 1947 I AUDIO SIGNAL SOURCE 1n TNVENTOR ROBERT G.SCHRIEFER OSCILLATOR ATTORNEY Patented June 6, 1950 UNITED STATES ATENT ()i-FiFliCiE FREQUENCYIMODULATION SYSTEM Robert G. Schriefer, Fort Wayne, .Ind., assignor, Qbygmesneassignments, to Farnsworthltcsearch 'Corporatioma corporation of Indiana Application February 1, 1947, SerialNo. 725,762

4' Claims.

'1 This-invention relates to frequencymodulation systems and particularly to systems-of this. character Wherethe carrier frequency-is changed-by frequency multiplication converts1thephase shift of the carrier wave .into a frequency-modula- .tion in accordance with the modulating signal. In. such'systems, in-order to avoid distortion, the

phase shift generally-is limitedtoa maximum of 30-degrees. ln general practice, the phase shift .is rarely inexcess of..25. degrees. -In order to pro- .ducethe desiredfrequency deviation of the carrier wave, it is necessary tomultiply thephaseshifted carrier wave by a factor .of several .thousand.

It-is commonly preferred in practice [to generate thecarrier wave .whiQh-is to be phase modulated by means of vcrystal controlled oscillators. About the lowest practicalfrequency.which can be-obtained fromapparatus-of this character is of the ordervof loo'kilocycles. In a'dditiomaccording to present Lfrequency allocations established by the .Federal Communications Commission, the frequency modulation channels are in the region of 100 -megacycles. .It .isseentherefore, .that if a 100 kilocycle carrier wave were to be multiplied in frequency bya factor of '100(), there would .be produced a carrier wave v:having afrequency of 10.0.,megacycles. il-Io'wever, such a relatively lowmultiplyingffactor Wouldproduce a frequency deviation atthe 100-. megacycle carrier frequency which wouldbe only a smallfracjtional partof the 75 kilocycle frequency deviation which the. Federal Communications Commission has ,established as standard. Consequently, when ausin priorart systems employinga single phasemodulatingsta a. it is not possible Joy a, strai ht multiplication process of the carrier Wave 'frequency .to develop a carrier wave having the frequency which mustberadiated andalso'h'aving the ,required :frequency deviation. ,In order to efiectthe. desired ,result,;therefore, it has'been necessary, .after some ,frequency multiplication,

to convert the frequencyemultiplied carrier wave to .a carrier wave of f lower "frequency ,which turnis.multipliedAinjrequencyistill more.

L'Lhe j'frequerxcy gconrersion, generally i is ge'fiectefi The frequency conversion, therefore, in the fiprst place, requires-the use of additional equipment. In the second-placeit is'necessary to :take suitable .precautions against the spurious -;beat rfrequencis created by the operation -of the frequency conversion apparatus. These precau- .tions generally consistnof the-provisionoftuned circuits, together with suitable shields for cer- .tainof :the apparatus. Here-again is additional apparatus which mustbeprovided.

The use of a heterodyne -frequency -converter also requires an additionalloscillator which, of course, preferably iscrystal controlled. Nevertheless, inasmuch as the systems must include at least two oscillators operating generally independently of vone another, there -is :a tendency forthe beat frequency-whicnzis produced to vary. Since the beat-frequency is employedinthemroduction of the frequency which :is to .be'radiated, there cannot be tolerated-any frequency variation thereofwhich is produced asraresult of thednconstancy of either one or both ofthetwo oscillators .employed to .produce .it. :Therefore, it .is common .practice to use frequency comparison and control apparatus between the (final .output circuit and .the oscillator .used for generating the .originallcarrier wave. Hereagain additional apparatus required for the successful operation ofsystems of .thelcharacter known in .the;prior .art.

Consequently, there isa need forya system by .vvhich'to effect phase modulation wherein amuch greater degree of phase shift .maybeproduced. Such a system would not require frequency multiplication by ,factors so great that ,heterodyne .frequency convertersare necessary.

, It is an .obiect ,of the present invention, therefore, to provide a novel method of .,freq11 ency .modulation, wherebyto obviateflthe stepofreducingthe frequency of a lphasee'shifted carrienwave after somefrequency multiplication thereof before additional gfrfiquency multiplication .is e!- ifected.

,A .further object of the invention is .to ,provvijde novel apparatus .for effecting .a phase shift of .a carrier ,frequency of considerably greater ,thann3l0zdegrees without, at the same time, in.- Ltrdducing distortion of the modulating signal.

Another object of otheinvention is to provide a novel .phase modulation system ,whereinseveral modulating, stages are employed to lefidctlan a gregate phase shift of considerably greater magnitude than,30 degrees.

According 'to present practi;ce, most phase shift by the use lofihfierofclynejreguQnqy converters. Il modulation systems require the use of several .the conductivities of the respective tubes. is produced a resultant quadrature voltage which,

tubes in the modulating stage. In some cases as many as five tubes are employed to effect the desired phase shift of the carrier wave frequency. In accordance with the multi-stage phase modulation feature of the present invention, several modulating stages may be coupled together to produce the greater total phase shift desired. However, such a system using prior art phase modulators requires a great deal of apparatus since the several tubes required for each modulating stage must be duplicated for each one of such stages. Apparatus of this character, while capable of operating satisfactorily in accordance with the invention, is relatively costly to build, operate and maintain. Therefore, there is a need for a simpler type of phase modulating apparatus wherein the number of tubes and other components is reduced to a minimum.

Therefore, it is another object of the invention to provide a novel phase modulating system comprising no more than two tubes and a relatively small number of other circuit components.

In accordance with the invention, there is provided a novel method of frequency modulation, the first step of which consists of producing a plurality of phase shifts of a carrier wave, each in accordance with a modulating signal. It is preferable that each of the phase shifts be of substantially the same magnitude. Another step of the novel method is the combining of these phase sh fts to produce a carrier wave which is shifted in phase in an amount equal to the total of all of the individual phase shifts, thereby eifecting a frequency modulation of the carrier wave having a frequency deviation dependent upon the number of phase shifts produced. Finally. if desired, the frequency of the phaseshifted carrier wave may be multiplied to effect a frequency modulation of the carrier wave having a greater frequency deviation.

Also in accordance with the invention, one form of apparatus for carrying out the novel 'method of frequency modulation comprises a plurality of phase shift modulation stages coupled in cascade to a source of carrier wave. Each signal impressed upon the tubes, the two quadrature components are balanced out against one another in the output circuit. The modulating signal is impressed upon the two tubes in opposite phase in a manner to control the respective conductivities of the tubes. The output circuits of the tubes are coupled together in like polarity.

As a result, the modulating signal oppositely varies the conductivity of the two tubes. With respect to the in-phase exciting component, there is reproduced in the output circuit of the tubes 'a corresponding component which does not vary in amplitude or phase with signal modulation. However, with respect to the quadrature exciting components, there are reproduced in the output circuit of the tubes, components which are 1S0v degrees relative to one another, and 90 degrees respectively relative to the in-phase component and which vary in magnitude in proportion to There when combined with the in-phase voltage, de-

By means velops an output circuit voltage which is shifted in phase from the output circuit voltage resulting from the zero modulation of the tubes.

The phase-shifted output circuit voltage is employed to excite the succeeding modulating stage, the two tubes of which are varied in their respective conductivities under the control of the modulating signal. A sufiicient number of such modulating stages are coupled in cascade to produce a carrier wave which is phase-modulated with sufiicient deviation that there is required a relatively small frequency multiplication to produce the desired signal frequency deviation. The output circuit of the last signal modulating stage is coupled to a frequency multiplier which, in turn, is coupled to an output circuit.

For a better understanding of the invention, together with other and further objects thereof, reference is made to the following description, taken in connection with the accompanying drawing, and its scope will be pointed out in the appended claims.

The single figure of the accompanying drawing is a schematic circuit diagram of a frequency modulation system, in accordance with the present invention.

Having reference now to the drawing, the phase modulating apparatus consists of two cascade-connected modulating stages. It will be understood that the present disclosure is merely for the purpose of illustrating the underlying principles of the invention. In broadcast practice, where relatively large frequency deviation is desired, more modulating stages will be required to elfect enough of a phase shift to obviate the use of heterodyne frequency converters.

'However, where more modulating stages are used, they may be coupled together and to the source of modulating signal in the same manner as the two stages illustrated herein.

The first phase modulating stage comprises two vacuum tubes I and 2 which, as illustrated, may be tetrodes. Similarly, the second of the illustrated modulating stages comprises two 'tetrodes 3 and 4. The modulating stages are excited initially from a source of carrier wave, such as an oscillator 5. The carrier wave is shifted in phase under the control of a modulating signal such as that derived from an audio signal source 6. The output circuit of the cascaded modulator is coupled to a frequency multiplier I which, in turn, is coupled to an output circuit 8 which may, for example, be the power amplifier of a transmitting apparatus.

More specifically, a tuned circuit 9 consisting of a coil l 0 and a shunt condenser I I is connected j.to the terminals of the oscillator 5 as an output circuit therefor. The circuit 9 is tuned to the oscillator frequency. Also, the control grids I2 and I3 of the first stage modulator tubes I and 2 respectively are connected to a tuned exciting input circuit I4 which consists of a coil I5 inductively coupled to the coil II) and a shunt condenser IS. The circuit I4 also is tuned to the oscillator frequency. Additionally, the oscillator output circuit is coupled by a condenser I! to a .center tap I8 on the coil I5.

The audio signal source 6 is connected to a coil IQ comprising the primary winding of a Another coil 2I forming the secondary winding of the transformer is inductively coupled to the primary coil I9 and its terminals are connected to the screen grids 22 and 23 of the modulator tubes I and 2 respectively. The cathodes of the tubes I and 2 are grounded and connected through a source-pf iSGIQeIl .sgrid azoitage, such as a battery 24, to a center tap on thesec- .ondary :coil 521i.

f-fEhe anodes iOf the two :tubes 1| and :2 {are 'iQOl'bnected :together :and to one terminal .of a .tuned the secondzstage modulating .tubest and 4, re-

spectively, are connected to the terminals of a timed excitingzinputcircuit .33 comprising ;;a .coil .3Land ashunt condenser 35. .This circuit also is tuned-to theoscillator frequency. Theanodes of the tubes I and .2 are coupled by a condenser 3.61 to acentertap 31 .on thecoil 34.

The audio signal sourced-also is connected to :a-primary-transformer winding 38 which is arranged in series with the first stage primary transformer winding 19. .A secondary transformer winding 39 which is inductively coupled to the primary winding 33 has its terminals connected-to the screen; grids 4i! and 14.2 :of :the second stagemodulating tubes 3 and 4, respectively. The grounded-cathodes .of the tubes 3 "and .4 are-connected through a source .OfzSCIGBl'l rid voltage such as abattery- 43 to'a center tap 445011 the winding 3.9. '.The routputgeircuitof the tubes 3 and a4 :includesia circuit 45 tuned :to the oscillator .ifrequencyand consisting of .a coil 46 and=za 'shuntcondenser M. Oneterminal of this output circuitfis connected to the anodes of the tubes andthe other terminal to the positive poleof a battery 48 which has its negative pole connected tOithG'CathOdGS' of the tube.

Another circuit-49 which is tuned to: the I oscillator frequency and which consists -.of :a coil 25:! and a shunt condenser :52 :is inductively coupled to the output circuit 45 of :thBrSfiCOIld modulat- .ing -stage. .connected tothe frequency' multiplier 'l.

:Batteries 1:24, :29, 44 .and A8 have beenrshown separatelyin' order to simpli y: the. drawing. :How- .ever, in accordance with well'Tknown-practice, a single power supply may *he employed, if .desired. Mutual isolation 'oi the several -.circuits maybe effected by the use -of blocking-condensers in a conventional manner.

Having reference new to operation. of the describedsapparatus, the ifunctienin .of only the first stage-(modulating tubes .-and 2iwill'be1considered in detail since the second stage --.tube s 3 and 4101381381136 in a similanm-anner. \By-meansaof the coupling condenser i-I'l the oscillator tirequency :is impressed =-.upon -:the-c0ntro1 grids AC2 aandrlt of the tubes I and2,*respectively in phase. Thisis the so-called in-phase: component'reierred .to. :Also, the oscillator irequencywis impressed upon the control grids of these tubes by .means of the inductivecoupling of the coils 1 Ill and J5. By-reason of :the coupling of the terminals. of-the .coil IE-tothe control grids-L2 and43,.respectively, the .tubes l and .-2 .are excited in-.;o pposite .phase. This is the .so-called quadrature scomponentareferred-to, byrmeansrof which :the tubes .areeX- -.cited in-apushepull -manner. .Also, by reasonsof the combination of this coupling and-the connec- .tion :of :the coupling .condenser I l to .the .center .iva |p"l8-.0f= :t'he;COi11| 5,.thequadratureexciting componentfforthertube. l ;is, ..at .a-,giv.en instance,.lead- The terminals of -.the circuit ".49 are i i6 ing'ztheiinepha'se scomponent'by-i90 degrees and; at the same instant, the quadrature componentimipressed zupon-tthe tube 12 .is Slagg'ing :the tin-phase component .byidegrees. As a consequence of thiscircuit arrangement, withno modulating signalsimpressed upon the tubes, they are of substantiallyequal conductivity. In these circumstances, the leadingand"lagging: quadrature componentsi'balan'ce oneanother out'inthe paralleled output-circuits of the tubes. There is, therefore, 'reproduced' in the-output: circuits of thetubes only 'theiin phase .exciting component.

Nowia'ssume tha't a modulating signal derived fromithei'source I6 is impressed upon these tubes. :In view :oi 'the push-pull "coupling :of the coil 21 .toithe screen' gri'ds F22 'and"23 respectively of the tllbESil and 2, themodulatingsignal is'impressed uponithesezitubestin :oppo'sitezphase. -:Consider, for :example, the instantaneous condition when the screen: grid '22 :is more positive st'han' it is in the abs'encezof themodulating signal. *Atthe same :time the screen gri'd 23 E'O'f the itu-beiisimore negativeEthamit iscwhenno' modulating signal is present'ibythessame 'amount tha't the screen "grid 5 22 'o'f'thetube l is more positive. .The conductivity of th'e :tuoe I "is increased "and-"the conductivity of the tube 2 is decreased by a like amount. As a result, the inephase component 4 of the exciting voltage which is impressed upon'theftube ii ,"is in- :creased 'in'zmagnitude in the 'output'circuit and them-phase exciting component, "which. is impressedupon-'the tube' z, is decreased in theout- "put circuit by "substantially the "same amount. However, the in-phase components which are reproduced inthe output circuits'iofthetubes com-- bine to produce a voltage 'of substantially the same ma-griitude as inthe case where no modulat- 'ing si'gnal is-present. Thus, no change 'is made inthe output circuit with respect to the in-phase excitin'g :component :with signal modulation ap- 'plied tothe tubes.

con'sidering now'the'quadrature exciting components assume that at a giveninstant'the control -grid-M ofthe tube"? isn'ega'tive. "The'leading 'quadrature component impressed upon the *tube "I is reproduced at increased magnitude in the output circuit. Simultaneously, thelagg'ing quad- "rature component'impressed upon the tube "2 is reproduced in the output circuit at decreased magnitude. It is seen, therefore, that underthese conditions the quadrature components do not balance'one another out inthe output circuit. In-

stead, there is produced a resultant quadrature component which leads the in-phase component by '90 degrees.

The voltage which is produced in the output circuits of the tubes is the result 'o'f" the in-phase -andthe quadrature'components. In the assumed "case'this resultant voltage is advanced in phase fromth'e resultant voltage which is reproduced when-no modulating signal is present. The phase angle of advancement of this resultant voltage is dependent upon the relative magnitudes of the "reproduced quadrature components which, in turn, are-representative of the amplitude of the modulating signal. By suitably proportioning the circuit components, the maximum angle of phase shift may be made no greater'than 30 degrees for per cent'modulation with low distortion. Dbviously, greater phaseshifts may be produced where relatively narrow band operation is desired and'signal distortion isnot-a primary consideration. With thepresent facilities, the phase angle can be varied over a range of plus or minus 80 degrees without difiiculty.

The phase-shifted voltage developed in the output circuit of the tubes l and 2, comprising the first phase modulating stage, is coupled to the tubes 3 and 4 of the succeeding phase-modulating stage in substantially the same manner as the oscillator 5 is coupled to the tubes of the first stage. Hence, effectively the successive stages of the modulating system are excited in a series or cascade arrangement. The modulating signal derived from the source 6, however is coupled by means of the transformer windings 38 and 39 to the screen grids of the tubes 3 and 4 in substantially the same manner as the audio signal source is coupled to the first stage tubes l and 2. Hence, the modulating signal is coupled to the successive stages of the modulating system effectively in a parallel circuit arrangement. The second modulating stage effects a further phase shift of the carrier wave so as to produce in the output circuit of the second stage a voltage which is shifted in phase from that derived from the oscillator 5 through an angle which is substantially equal to twice the angle of phase shift capable of being produced by either one of the modulating stages.

It is evident that, by providing a sufiicient number of modulating stages such as the two described coupled together in cascade with respect to the exciting voltage and in parallel with respect to the modulating voltage, a total phase shift may be produced which, in magnitude, is approximately equal to the product of the phase shift of one stage and the number of stages. Ordinarily five or six of such cascaded modulating stages are sufficient to produce an adequate phase shift of the carrier wave so that only a relatively small amount of frequency multiplication is recuired to produce the desired frequency deviation in the output circuit. In this manner there is no need to employ apparatus such as heterodyne frequency converters in order to effect the frequency deviation desired. It is apparent, furthermore, that the provision of a relatively simple phase modulating circuit minimizes the apparatus required and makes it feasible, as well as economical, to employ a number of such modulating stages.

While there has been described what, at present, is considered the preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it, therefore, is aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. A frequency modulation system comprising, a source of carrier wave, a plurality of phasemodulating stages, means including said carrier wave source for exciting the first of said modulating stages both with. an in-phase carrier wave component and with quadrature out-of-phase carrier wave components, means including the respective immediately preceding modulating stages for exciting the others of said modulating stages both with an in-phase carrier wave component and with quadrature out-of-phase carrier wave components, a source of modulating signal, means including said modulating signal source for effecting a phase modulation of the respective exciting carrier waves of all of said modulating stages, and means for multiplying the frequency of the phase-modulated carrier wave derived from the last one of said modulating stages.

2. A frequency modulation system comprising, a source of carrier Wave, a plurality of phasemodulating stages coupled together in cascade, said modulating stages being substantially similar and each having an exciting input circuit, a modulating input circuit and an output circuit, means including said carrier wave source for impressing upon the exciting input circuit of the first of said modulating stages both an in-phase carrier wave component and quadrature out-ofphase carrier wave components, means for coupling the exciting input circuits of each of the others of said modulating stages to the output circuits or the immediately preceding modulating stages to excite each of said other stages both with an in-phase carrier wave component and with quadrature out-of-phase carrier wave components, a source of modulating signal, means including said modulating signal source for impressing a modulating signal upon the modulating input circuits of all of said modulating stages, a frequency multiplier, and means for coupling the output circuit of the last one of said modulating stages to said frequency multiplier.

3. A frequency modulation system comprising, a source of carrier wave, a plurality of phasemodulating stages coupled together in cascade, said modulating stages being substantially similar and each including two vacuum tubes, each of said tubes having an exciting input circuit, a modulating input circuit and an output circuit, means for coupling the exciting input circuit of the first of said modulating stages to said carrier wave source to excite said two tubes with an inphase carrier wave component and with quadrature out-of-phase carrier wave components, means for coupling the exciting input circuits of each of the others of said modulating stages to the output circuits of the immediately preceding modulating stages to excite said other stages both with an in-phase carrier wave component and with quadrature out-of-phase carrier wave components, a source of modulating signal, means for coupling said modulating signal source to the modulating input circuits of all of said modulating stages, a frequency multiplier, and means for coupling the output circuit of the last one of said modulating stages to said frequency multiplier.

4. A frequency modulation system comprising. a source of carrier wave having an output circuit, a plurality of phase-modulating stages coupied together in cascade, said modulating stages being substantially identical and each including two vacuum tubes, each of said tubes having a cathode, an anode, a control grid and a screen grid,

each of said modulating stages also having an exciting input circuit including said control grids, a modulating input circuit including said control grids and an output circuit including said anodes and said cathodes, means for coupling the exciting input circuit of the first of said modulating stages to the output circuit of said carrier wave source to excite said two tubes with an in-phase carrier wave component and with quadrature outof-phase carrier wave components, means for coupling the exciting input circuits of each of the others of said modulating stages to the output circuits of the immediately preceding modulating stages to excite said other modulating stages both with an in-phase carrier wave component and with quadrature out-of-phase carrier wave components, a source of modulating signal having a plurality of output circuits, means for coupling the respective output circuits of said modulating signal source to the modulating input circuits of all of said modulating stages, a frequency multiplier having an input circuit, and means for coupling the output circuit of the last one of said modulating stages to the input circuit of said frequency multiplier.

ROBERT G. SCI-IRIEFER.

REFERENCES CITED Number 10 UNITED STATES PATENTS Name Date Affel Aug. 25, 1925 Carpe Feb. 18, 1930 Llewellyn Sept. 12, 1933 Crosby Apr. 5, 1938 Mountjoy Nov. 8, 1938 Bliss Nov. 5, 1940 Crosby July 22, 1941 Usselman June 13, 1944 Stodola Mar. 2, 1948 

